Many appliances contain an automatic ice maker that deposits ice into a storage bin. The function of an ice maker is well-known in the art, so a detailed description thereof is not provided herein. An ice maker is activated to produce ice and deposit the produced ice into a storage bin. As long as an ice level sensor determines that the storage bin is not filled to a threshold (i.e., full) level, the ice maker continues to deposit ice into the storage bin. The ice maker is deactivated when the ice level sensor determines that the storage bin is full. In this way, the ice level sensor provides an interlock to avoid overfilling of the storage bin.
One common ice level sensor is in the form of a mechanical arm or bail. The normal resting position for the bail is in the space where ice will accumulate when the storage bin is full. When the bail is pushed upward by ice or prevented from being moved downward to the normal resting position, due to the ice level reaching or exceeding the full level, an indication will be provided to the ice maker to deactivate the ice maker. Mechanical sensors of this type frequently suffer from failure due to normal wear and tear of moving parts and damage resulting from food items being placed in the ice storage area or from removal and replacement of a removable ice storage bin. Mechanical arms are also prone to freezing in a non-interlock position while the ice maker continues to operate.
During the ice making process, ice cubes can become partially melted and then refreeze, being frozen to adjacent ice cubes. This may occur when an ice cube tray is heated slightly to allow cubes to be easily removed from the tray in the ice maker, and can also occur due to the warming cycles in a frost-free freezer. The top layer of ice cubes in the storage bin may be frozen together, forming a rigid layer that does not collapse even when ice beneath the rigid layer is removed from below. This condition is referred to herein as ice bridging. Ice level sensors that detect the presence of ice at the top of the storage bin, such as the mechanical bail, provide a false full indication when ice bridging occurs because the sensor continues to detect the presence of the top layer even though the ice bin is otherwise empty. Ice cubes sticking together can also prevent even lateral distribution of ice within the storage bin. In such situations, ice may be present at the resting position of the ice level sensor bail even though the storage bin is not otherwise full. This can occur, for example, when ice is removed from only one end of a storage bin or when new ice deposited into the storage bin is not evenly distributed.
Other technologies used for detecting the level of ice in a storage bin include ultrasonic sensors, temperature compensated infrared sensors, load cells, optical sensors and capacitive detection sensors. Ultrasonic, optical and temperature compensated infrared sensors are non-contact sensors (i.e., require no physical contact with the ice) that use transmitters and receivers to detect whether the storage bin is full, but they are generally designed to detect the ice level at only one position in or above the storage bin, which can lead to a false full indication during ice bridging or unevenly distributed ice conditions. In addition, fogging and/or frosting of the transmitters, receivers and/or light path, can interfere with operation of light based sensors.
Improper operation of mechanical and non-contact sensors alike can be caused by frost build-up around the sensor, such as by freezing of level contacts or interfering with suspension of a storage bin where a load cell is used. One method for dealing with this problem is the addition of heaters around the sensor parts, but heaters increase the cost of the system and use more power. Other solutions can be significantly more expensive. Load cell sensors and ultrasound sensors both suffer from unbalanced loading of ice within the bin.
Increasing the level of accuracy of an ice level sensor is accomplished by detecting the level or presence of ice at more than one location in the storage bin. However, duplication of parts, such as bails, transmitters and receivers and control circuitry increase the cost of the system.
Capacitive sensing technology is based on detecting changes in the capacitance of an electrode in a circuit. The capacitance measured on an electrode depends on the dielectric constant of the space around the electrode through which an electric field passes. Projected proximity capacitive sensing allows the electrodes to be at a distance from the medium to be detected. Since there is a difference between the dielectric constants of air and ice, capacitive sensing technology can detect differences in the amounts of ice and air within a distance of the electrodes. However, the accuracy of such detection can be affected by frost that builds up on the electrode(s) or on the walls of an ice storage bin within a detection space of the electrode.
Reed et al. (U.S. Pat. No. 5,460,007) discloses an ice level sensor that relies on capacitive sensing. Specifically, the sensor disclosed by Reed et al. uses an analog controller based on a Wheatstone bridge to detect a difference between a first measured capacitance and a second measured capacitance. The first measured capacitance is the capacitance measured between a first electrode and a second electrode (ground electrode) positioned adjacent to the top of an ice storage bin. The first measured capacitance changes if there is ice in the detection space of the first electrode. The second measured capacitance is the capacitance measured between a third electrode and the second electrode (ground electrode). The third electrode is positioned above the ice storage bin. The second measured capacitance is based on the dielectric constant of the space above the ice storage bin, which is an air space. Reed et al. compares the measured capacitance of the first electrode to the measured capacitance of the third electrode to determine whether the portion of the ice storage bin contains ice or air. This detection device is limited to a single point of detection in the portion of the ice storage bin that is near the first electrode. Due to the configuration of a Wheatstone bridge, the number of electrodes that can be monitored by a controller of this type is fixed. In addition, the detection accuracy of the analog controller depends on circuit balancing that may be difficult to recreate in a manufacturing environment and/or may require calibration, which adds to the manufacturing time and cost.
Therefore, what is needed is an ice level sensor that does not rely on moving mechanical parts, will not be affected by fogging, will not be expensive or difficult to manufacture, does not require calibration, will provide detailed information about the quantity of ice in a removable storage bin with a high degree of reliability and will dynamically adjust to frost build-up over time.